The proposed work is a continuation of our previous and current studies on the regulation of the enzymes involved in the catabolism of galactose in yeast. This system is being used as a model system for such regulation in eukaryotic microorganisms. Eukaryotic features of the yeast cell, notably their possession of mitochondria and their genetic organization, make yeast an ideal organism to study regulatory phenomena in eukaryotes that cannot be as easily or inexpensively done in higher eukaryotes. Our current and future work is concentrated on defining the molecular role of the known regulatory loci in the system on the regulation of the synthesis of the structural elements of the system. In addition, data from this system are being compared to comparative data derived from similar studies on the regulation of the synthesis of melibiase and maltase in the same strains of yeast. The effects of different carbon sources and cultural parameters on the synthesis of these enzymes in wild type and mutant strains are being investigated. A major part of our effort is designed to clearly define the relationship between the ability of different strains to utilize different sugars when the cells are respiratory deficient, i.e., incapable of oxidative metabolism. There exists in different strains striking differences in their abilities to use different carbon sources: these differences are influenced by the respiratory state of the cell, whether altered genotypically or phenotypically, and the presence or absence of both previously detected regulatory loci and at least two new loci detected by genetic crosses among different strains. Future work is planned to precisely define first the genetic nature of these loci, their phenotypic effects, and finally their molecular site(s) of action. These new loci and their relationship to the respiratory state of the mitochondria may represent a mode of regulation of catabolism unique to the eukaryotic level of cellular organization.